The invention relates to an air-mass sensor, a process for the calibration of the air-mass sensor and a signal evaluating process of the air-mass sensor.
Air-mass sensors or air-mass meters are used particularly to determine the intake air flow of an internal combustion engine. Heating element anemometers, also designated thermal air-mass meters of known type, are used for the design of air-mass sensors. Two sensors or probes, one of which determines the temperature of the intake air and the other of which is heated to a specific over-temperature relative to the ambient temperature, are connected in different branches of a Wheatstone bridge circuit. The heated probe serving as the measuring probe is cooled by the current of air, dependent on its velocity and temperature. The additional amount of energy or a quantity dependent thereon that is required to maintain the adjusted over-temperature is then a measurement of the air-mass that has passed through the sensor.
DE 43 31 772 C2 discloses an air-mass meter, a process for determining a flowing quantity of air and a device for determining an electrical output of the air-mass meter. The air-mass meter has a temperature probe resistance and two heating elements in the form of electrical resistances, which are connected with other resistances in a bridge circuit. The temperature probe resistance and the first heating element form an air-quantity detector and are arranged on a common base plate in an air-current path. The second heating element is a component of a control circuit and is separated from the air-quantity detector in the air-current path. At a reference resistance, which is in a separate detection device, an electrical current signal determined from the bridge circuit is converted into a voltage, digitized, and input into a microprocessor of the detection device. The microprocessor balances differences between a voltage at the reference resistance and a standard value based on stored information, in routine air-mass detection, in order to eliminate the effect of the tolerances of the reference resistance. The control for the fuel injection is then produced by the electrical output current signal that is determined.
It is a disadvantage that the air-mass meter and the detection device secondary to this must be synchronized each time to one another. Also, with only a single air-mass meter, the direction of flow of the air mass cannot be detected.
An object of the invention is to provide an air-mass sensor and its mode of operation in such a way that expensive calibrations relative to the engine are not necessary and the direction of flow of the air mass can be detected simultaneously with only one air-mass sensor.
The invention is based on the concept of creating an air-mass sensor as a compact air-mass sensor module in a simple construction, with which, in addition to determining the quantity of a flowing air mass, its direction of flow can also be detected and which can also be manufactured as a mass-produced product. This objective is achieved by a construction of the air-mass sensor which has a measurement element or a sensor element, on which two separately acting heat sensors and two temperature sensors are disposed, which are connected at least to two separately operating bridges, which are also disposed in the air-mass sensor module. These two bridges make possible the determination of the quantity of the air mass and/or the direction of flow of the air mass.
Further, by integrating a microprocessor into the air-mass sensor module and with a selected programming thereof, the signal processing of the bridge signals that are determined takes place in this microprocessor, whereby a higher measurement accuracy, a greater flexibility, and a broader functional range of the air-mass sensor module are obtained. By means of support places in tabular form filed in the microprocessor and an individual signal processing, it is possible to synchronize the air-mass sensor module independently of the sensor tolerances to a specific series of an engine model. In this way, in the production of the air-mass sensor, a series of sensors can be obtained which have the same signal magnitudes at their interfaces, and thus are calibrated to a certain extent. The different sensor tolerances are equilibrated by measurement of correction values determined in a base mode or calibration mode, whose values are entered into the support place tables of the microprocessor. In order to obtain a self-contained air-mass sensor module, necessary trimming of the resistances for the calibration of the heat sensor deviations and necessary load resistances of the bridge circuits are integrated into the module.
The temperature sensors and heat sensors are introduced into two separate membranes, which are applied onto a substrate of the measurement element. The membranes are preferably disposed in one plane on one side of the substrate.
By introducing very thin membranes onto the substrate, the outflowing heat is kept small and the thermal inertial mass is minimized. Short activation times as well as high dynamics of the measurement element are achieved by the small heat capacity of the membranes and the heating structure. These dynamics are advantageous for application in mass-flow measurements with reverse or backflow.
A temperature transfer between the heat sensors and from these to the temperature sensors is prevented by a suitable selection of the distance between the heat sensors and the temperature sensors, whereby measurement errors are avoided.
The membranes protect the embedded sensors from chemical attack. Thin membranes also make it possible for the heat sensors as well as the temperature sensors to have the same thermal response times.
Preferably, silicon is used as the substrate and silicon dioxide as the membrane. The utilization of silicon technology makes it possible to reduce the resistance values of the heat sensors and the temperature sensors, whereby also the size can be kept small, despite the condition that in each bridge there must be one bridge branch of higher electrical resistance than in the other branch. This condition is satisfied by providing a voltage reduction by additional means in the temperature bridge branch of each bridge. In the case of a lower supply voltage obtained thereby, consequently, the resistance value of the temperature sensor can be reduced, whereby also the dimensions of the temperature sensor are reduced. This same reduction in voltage is also produced in the other bridge circuit with the heat sensor, the heat bridge branch, in order to retain an equilibrium of the bridges. This reduction is also produced in the second, separately acting bridge.
For further heat dissipation and thus for preventing a reciprocal temperature influencing of the heat sensors and the temperature sensors, the heat is also released to the environment through cross-pieces on the substrate. For this purpose, the substrate is adhesively secured to a substrate carrier that is a good heat conductor, preferably a ceramic piece. The ceramic piece itself is adhered by means of a heat-conducting adhesive, which is filled with small beads of defined size, onto a cooling unit, preferably an aluminum cooling unit, which releases heat to the environmental air by means of ribs. The beads also have the objective of assuring a uniform thickness of the layer of heat-conducting adhesive from one module to another to provide a specific high tolerance as well as a uniform heat dissipation.
Since a cooling unit is already included on the substrate with the measurement element, the temperature of the substrate is held at the temperature of the intake air by the cooling unit, whereby the already-mentioned thermal effect between heat sensors and temperature sensors is additionally minimized.
This measurement element with substrate carrier is mounted in the flow opening of a housing of the air-mass sensor module such that an attachment or clamping on all sides is produced in the housing, whereby the flowing medium is guided past or over the measurement element on one side thereof. By embedding the measurement element in the housing, this measurement element is protected from breakage and thus from disruption and it also has a robust, but simple structure. The measurement element of this measuring device is also protected against damage from air flow, since the measurement element is disposed flush with the flow opening of the housing, and a substrate carrier is surrounded by a frame.
The microprocessor is a component of the electronic part of the air-mass sensor module and is accommodated on a hybrid unit. The microprocessor is connected with the measurement element by means of conducting connections, for example, strip conductors. Signals are measured and entered into the support place tables in the calibration mode of the microprocessor, whereby measurement points, which correspond to the characteristics required by the customer, are filed in the support place tables. The signals are thus initiated as crude bridge signals. A comparison between the bridge output signals and the theoretical signals is then produced for the measurement points of the required characteristic curve, whereby an entry can be made into the support place tables for each established measurement point.
After the complete determination of the measurement values or the correction values and the filling in of the support place tables, another programming of the microprocessor is conducted with the signal-processing software for the air-mass sensor module. The microprocessor is brought into the programming mode for this purpose. The signal-processing software to be programmed can be partially supplied in advance by the customer, but it can also be determined, for example, by tests.
The determination of an output signal interpreting the air-mass quantity by means of measurement values filed in the support place tables, also designated as absolute values or the correction values, is made in a signal evaluating path within the signal evaluation. In addition, a determined temperature signal is output by means of another signal evaluating path. This temperature signal can be utilized as information relative to the external temperature of the intake air and thus, for example, used for regulating the heat of a passenger compartment. There is also produced a better determination of the fuel injection, since the output signal of the first signal evaluating path, which interprets the air-mass quantity, is additionally post-compensated for temperature compensation by means of adapted bridge resistances of the front bridge in the software via output-signal support places and temperature support places of the support place tables.
The software for processing the signals can be defined prior to running the series.